Good topic. I think you'll have fun on the 250. I race a 250 in supermoto and have a great time. It's even more sweet when I spank half the 450s good laughs. With the right mods, the 250 is a lethal weapon in the corners. If you carry more corner speed and get on the gas early it's very difficult for the 450's to get corner position on you. I often would like to have my own 450, but for now I'm having so much fun. I'm 40 210 lbs and definitely not over the hill
Are you going to race this in supermoto? Stay stock springs and valves. A good increase will come with good head porting, cam timing, high compression ratio, balanced crank, and proper fuel/jetting. First ask your self how much power can you use and where do you want the power to be delivered. Then apply the right mechanical solution to your needs. I suggest just go with what you have now, race/practice a while and go from there.